A. Field of the Invention
The present invention relates to voice band data transmission and, more particularly, to a system and method for implementing an end-to-end error-correcting protocol in a voice band data relay system.
B. Description of the Related Art
The Telecommunication Standardization Section of ITU (ITU-T) has defined procedures for duplex modems operating on the general switched telephone network and on leased telephone-type circuits. ITU-T Recommendation V.42 describes error-correcting protocols for use with V-Series duplex Data Computer Equipment (DCE) to accept start-stop data from the Data Terminal Equipment (DTE) and transmit in synchronous mode. Recommendation V.42 is based on providing data communication using a V-Series modem using a Public Switch Telephone Network (PSTN) or a digital network capable of carrying a digital representation of an analog signal, such as pulse code modulation (PCM). Recommendation V.42 contains an HDLC-based protocol referred to as the Link Access Procedure for Modems (LAPM). Additionally, an alternative procedure is defined in Annex A of the Recommendation. Compliance with Recommendation V.42 requires implementation of both these protocols.
FIG. 1 shows a system by which two voice band data modems, telephones, or facsimile machines communicate with each other, whereby a narrowband network is utilized in the communications path between the two data modems, telephones, or facsimile machines. In order to support V-Series modem connectivity over a digital narrowband network, the analog signal can be demodulated by a first gateway and the demodulated data can be transmitted over the narrowband network by the first gateway to the second gateway. The demodulated data can be re-modulated at the second gateway.
Referring to FIG. 1, a data terminal or computer 13-1 is communicatively connected to a first voice band data modem 12-1, which is communicatively connected to a first private branch exchange (PBX) 14-1, which in turn is communicatively connected to a first gateway processor 15-1. A telephone 10-1 and facsimile machine 11-1 are also communicatively connected to the PBX 14-1. The PBX 14-1 may alternatively be a PSTN or a plain old telephone system (POTS). The first gateway processor 15-1 provides connectivity with a narrowband network, which is shown as digital network 16 in FIG. 1.
Voice band data is typically transmitted over conventional analog telephone lines, such as those used by a PSTN or PBX, using 64 bit-per-second pulse code modulation (PCM). The PCM-modulated fax data (analog data) needs to be converted to a different form, that is, into digital data, by the first gateway 15-1, so that the digital data can be transmitted over the digital network 16.
A second gateway 15-2 provides a communicative connection to a second PBX 14-2, which provides a communicative connection to a second voice band data modem 12-2. Like the first PBX 14-1, the second PBX 14-2 may alternatively be a PSTN or a POTS. The second gateway 15-2 receives digital data from the digital network 16, and converts the digital data into analog data having a PCM format. The PCM data is then sent over the second PBX 14-2, whereby it is received by the second data modem 12-2.
The digital network 16 is typically of a narrower bandwidth than the analog portions (PBX 14-1, 14-2) of the communications path between the first and second data modems 12-1, 12-2. The digital network 16 may be a packet network or switched network, but could be any bandwidth limited or low rate communication system. The bandwidth limitations of the network 16 may be the result of congestion or other circumstances.
Typically, voice band data modems are capable of negotiating a common modulation/re-modulation scheme and data rate. In order to achieve inter-networking between different modems, additional protocols for call discrimination and modem type discrimination are employed. One method is to establish the same modulation scheme and data rate at both gateways to the analog network. Once the end-to-end modulation scheme has been established and a full duplex digital bit-pipe exists between the two gateways, the V.42 error-correcting protocol can be established transparently between the two modems via the bit-pipe. Currently, there are two techniques for establishing end-to-end V.42 error-correcting protocol when employing voice band data relay. These methods are described below.
The first method is based on local implementation of the Recommendation V.42 protocol (LAPM and Annex A) at the gateways, and transmission of the user information over the digital network via another HDLC based (or transport layer) protocol to ensure the data integrity. In this method the V.42 protocol is established between each data modem and the gateway to which the modem is connected. This approach can ensure that the data transfer over the link (i.e. the narrowband network) is optimized since non-informational data associated with the V.42 protocol can be removed by the gateways and not transmitted over the digital network. In addition, the gateways at each end can use flow-control to ensure that the modem data rate not greater than the available network bandwidth. The main disadvantage of this approach is that the complete Recommendation V.42 and Annex A protocols must be implemented at the gateways, and the memory required to implement these protocols may be greater than the available resources in the gateways.
A second method is transparent transmission of the demodulated data between the gateways. In this method, the V.42 protocol is established transparently between the end-to-end data modems without the knowledge of the relay gateways. This method can only be implemented if the network delay is less than 350 ms in one direction, as a result of a timer T400, defined in Recommendation V.42 (although, the timer T400 has a default value and is defined to be configurable, in most cases the value cannot be configured by the user). The relay gateways cooperate in establishing an end-to-end bit-pipe by selecting a common modulation/demodulation scheme. Following establishment of the bit-pipe, the data is transported across the link without protocol interpretation by the gateways. The gateways may reformat the user data into packets and add control and timing information, but implementation of the V.42 protocol at the gateway is not required. The advantage is a simple implementation, which requires less resources at the gateways. In addition, there is no need to have the V.42 layer implemented at the gateways.
The disadvantage is that the V.42 protocol will not be established if the network delay (end-to-end) is greater than 750 ms as controlled by the V.42 T400 timer (MNP protocol which is defined in V.42 Annex A has more a relaxed requirement but is also limited to a set timeout) Another disadvantage is that the established end-to-end modulation rate must be less than or equal to the channel rate (i.e. the throughput of the narrowband network). This disadvantage becomes apparent when demodulating and re-modulating a V.32 signal over an 8 kbps channel. V.32 modems have two rates of operation, one is 9600 bps and the second is 4800 bps. When the channel rate between the gateways is less than 9600 bps, the V.32 rate selection between the end-to-end is set to 4800 bps. This results in a low channel utilization—in this case the channel utilization is 4800/8000 or 60%.
Yet another disadvantage of this second method is that non-informational messages such as “Inter-frame fill” flags (defined in Recommendation V.42) are transmitted over the narrowband network. Inter-frame fill flags are transmitted between the end-to-end modems when there is no user data to be transmitted. Yet another disadvantage of this second method is that there is no mechanism to perform flow control locally between the gateway and the data modem at each end. Since the demodulated user data is at a constant stream, any fluctuation in the transmission rate over the narrowband network (e.g. a packet network) may result in loss of user information. Although jitter buffers can be used to provide a limited remedy for packet jitter in the network, there is no specific mechanism to flow control the data to match the effective network throughput.